<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <div id="my-id">
    <img alt="萝卜" id="inside">
    <div class="lonely"></div>
    <div class="outer">
      <div class="inner"></div>
    </div>
  </div>
</body>
</html>

<script>
  let firstArr = document.querySelectorAll("#my-id div div");
  console.log(firstArr);//NodeList(1)
  let secondArr = document.querySelector("#my-id").querySelectorAll("div div")
  console.log(secondArr);//NodeList(3)
</script>